The present invention relates generally to data communications, and more particularly, to a system and method for suppressing impulse noise in a multipoint communications environment using a linear block interleaver.
In the field of data communications, a modem is used to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems or other devices to communicate rapidly large amounts of data. Devices communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are well known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, differentially encoding the information to reduce errors and improve throughput, and transmitting the information over a communication channel. The constellation can include both analog and digital information or often merely digital information. At a receiver, the signal is received, equalized, and decoded in accordance with techniques that are known in the art.
In the above mentioned communications system, a control device, i.e., one that is located at a telephone company central office, connects the communication channel to a plurality of remote devices typically located at a customer residential or business location. This topography is known as a multipoint communication environment because one control device is communicating with a plurality of remote devices co-located at a remote location, over a single communication channel.
The communication technique between the control device and the remote device is generally half duplex in nature, meaning that only one device may transmit at any particular time. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two devices at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple devices to the same communication channel.
In any communication environment, noise is an ever present obstacle to optimal receiver performance. Noise imparted by the communication channel can be substantially eliminated through the use of well known techniques, such as precoding and channel equalization. Local noise imparted to a channel, such as periodic impulse noise, from local sources, such as electrical appliances and light dimmers, and random impulse noise, such as switching relays in a central office (CO), present an even greater problem that can degrade receiver performance.
All DSL equipment is susceptible to these impairments. In order to combat crosstalk, carrierless amplitude/phase (CAP) modulation uses a precoder, while discrete multi-tone (DMT) selectively disables the affected frequency bins. The 60 Hz periodic impluse noise (for example, that generated by a local electrical appliance such as a light dimmer) and other impulse noise generated for example by a switching relay at the central office is allowed to exist and the resulting errors are corrected by a Reed Solomon (RS) forward error correction code, sometimes in combination with a bit-wise interleaver. This arrangement works fairly well in point-to-point applications where the delay caused by the RS coding is not problematic, however, for multipoint applications, the RS and DMT codes are ineffective. For example, the precoder used with CAP is located in the transmitter, and will cause discontinuities when the transmitter is switched on and off, as frequently occurs in a multipoint environment. Furthermore, the control device in the multipoint environment can have but one set of precoder coefficients for broadcasting to all remote devices, which may have different precoder needs. Also, ringing caused by on/off polling cannot be canceled by a precoder because the transmitter and precoder are inoperable during the ringing transient.
Other error correction codes are available which can be used to reduce some of the errors caused by impulse noise. For example, block codes, and more specifically, linear block codes have been developed for use in communications systems to correct or reduce the number of burst errors in data transmission. However, these codes have excess delay and have been unable to successfully reduce or eliminate the type of random impulse noise generated by relay switching equipment in a telephone company central office.
Therefore, it would be desirable to provide a noise suppression system and method in both a transmitter and a receiver to reduce or eliminate the impulse noise imparted to a receiver, without the need for forward error correction.
The present invention provides an improvement to a communication environment by enabling a communication device to efficiently suppress impulse noise. This noise can be for example, switching relay noise imparted to the communication channel at a telephone company central office.
The present invention provides a system for noise suppression, the system comprising first logic, located in a transmitter of a communications device, configured to combine multiple symbols from a signaling constellation into n-symbol combinations and logic configured to transmit the n-symbol combinations to a receiver. At a receiver the linear block interleaver includes logic configured to receive the n-symbol combinations and second logic, located in the receiver of a communications device, configured to combine the n-symbol combinations in order to recover the original multiple symbols.
The present invention can also be conceptualized as a method for noise suppression, the method comprising the following steps. First, in a communications device, multiple symbols from a signaling constellation are combined into n-symbol combinations. The n-symbol combinations are then transmitted to a receiver. At a receiver the n-symbol combinations are received and combined in order to recover the multiple symbols.
The invention has numerous advantages, a few of which are delineated hereafter, as merely examples.
An advantage of the linear block interleaver is that it substantially reduces or eliminates impulse noise generated in the vicinity of a DSL receiver and elsewhere in the associated communication channel.
Another advantage of the linear block interleaver is that it has low throughput delay.
Another advantage of the linear block interleaver is that it is compatible with decision feedback equalizers and trellis coders.
Another advantage of the linear block interleaver is that it is low in complexity, simple in design, reliable in operation, and its design lends itself to economical mass production in communication devices.